Step 1. In a bowl blend together flour, butter, and salt. With your fingertips or a pastry blender (or pulse in a food processor) crumble butter until the mixture resembles coarse meal with some roughly pea-size butter lumps.
Step 2. Add sugar, baking soda (mixed with a teaspoon of lemon juice), egg yolks, and lemon zest. Use the sour cream to mix the ingredients and form the dough. Do not overwork the dough. The dough should be smooth, soft, and flexible. If the dough is sticky, add more flour; if it is too hard, add more sour cream.
Step 3. Ground the walnuts to your liking. Add sugar, hot milk, and the lemon zest. Mix and set aside.
Step 4. Break enough dough and roll out a circle about 9 inches in diameter. Once rolled out, cut the circle into eight slices like you would a pizza (see photo below). Add the walnut mixture to the widest part of the slice. Beginning at that end, roll the dough and mixture into a croissant shape. Place on an ungreased cookie sheet. Continue the process with the remaining dough.
Step 5. Bake at 350° for 9 to 12 minutes or until lightly brown. Sprinkle with powdered sugar.
Note: Another popular filling for cornulete is plum or apricot jam.